Network expansion is motivated by the prospects of new applications requiring a much higher capacity than that required by today's applications and is facilitated by the abundance of data transport capacity (often called bandwidth) of the optical telecommunication's medium. The realizable capacity of a telecommunication network is virtually unlimited. A network structure that enables virtually unlimited expansion while providing a high service quality is desirable and its introduction is overdue.
One of the tempting options to provide a high-capacity wide-coverage telecommunication network is to build on an existing structure. Two great network structures with global coverage already exist. The first is the almost flawless global telephone network. The second is the Internet. Neither, however, is a candidate for expansion to the needed capacity or capability. The telephone network's rigid circuit switching and huge infrastructure make it almost impossible to upgrade to carry data at high rates. The structure of the current Internet prohibits its growth without tremendous complexity and expense. This is further complicated by the unduly complex protocols that are an accumulation of patchwork performed since the Internet's inception.
In the pursuit of simplicity, one must resist a common ill-conceived notion that bandwidth is inexpensive and, hence, may be squandered to simplify control. The fallacy of this belief is twofold. Firstly, squandering bandwidth to solve a problem shifts the problem somewhere else because, eventually, the data content has to be sifted to retain the required information and, secondly, while the cost of communicating a data unit may drop significantly, the volume of communicated data is also likely to increase significantly.
Network Basics
In its simplest form, a network comprises a large number, P, of dual access ports. A dual port consists of an input port and an output port. A network user can access the network through a single port or through more than one port, and a port can support several network users. The ports can have different capacities, each operating at a different bit rate, for example a 2.5 Gigabits a second (Gb/s) or 10 Gb/s bit rate. The network user need not be aware of the network structure beyond the access ports.
The network provider's problem is to interconnect the P ports. Establishing a permanent path from each port to each other port is clearly unrealizable when P is a large number; 10 million for example. Interconnecting the P ports by a central core node is also unrealizable for a large value of P. For a practical solution, the ports can be divided into port groups, and each port group forms a node, called an edge node. An edge node consists of a source node that receives traffic from the input ports and a sink node that delivers traffic to the output ports. The source node and sink node preferably share memory and control. To form a network, the edge nodes can be interconnected directly or through core nodes, each core node would have a substantially higher capacity than a typical edge node.
The outer capacity of the network is the total capacity of its P ports. The outer capacity is the capacity available to the network users. For a given outer capacity, the use of a large number of low-capacity edge nodes has the advantage of reducing access cost, by virtue of the resulting proximity of the edge nodes to the network users, and reducing nodal complexity, by virtue of the node size. For a large network, with the number of ports P exceeding tens of thousands, the disadvantages of using low capacity inter-connecting nodes outweigh the advantages. While the nodal complexity is reduced, the network complexity is increased. Nodal complexity relates to components that are contained in a box. By contrast, network complexity relates to network elements that may be widely distributed, thus stifling control. The limited adjacency of a small-capacity node necessitates multiple hops for port pairs, hence requiring complex routing protocols. The use of multiple hops also increases cost and degrades performance as nodal delays and data losses accumulate. The use of multiple-hop routes from one port to another also complicates the route selection process. Consequently, network reliability is degraded due to the difficulty of determining alternate routes in response to route failure.
In a large-scale network, the number of alternative ways of interconnecting the edge nodes is enormous. To explore some of the alternatives, it is helpful to consider edge nodes of equal size, allocating a number, Q, of ports to each of V edge nodes, such that Q×V=P.
The Q ports of an edge node form an intra-nodal network, the complexity of which increases with the number of ports Q. The intra-nodal complexity is, however, contained in a box where the communication among its components is virtually instantaneous and control is, therefore, simplified. Furthermore, the nodal complexity has no operational implications and does not extend beyond the manufacturing phase. The number Q should, therefore, be limited only by optimal-access requirements and design limitations. High-capacity nodes of substantially reduced complexity can be constructed using a rotator-based architecture, which enables the construction of an edge node of 100 Tb/s capacity, with Q=10000 and a port capacity of 10 Gb/s (10 Gb/s input, 10 Gb/s output). A rotator-based switch is described in U.S. Pat. Nos. 5,168,492 and 5,745,486 issued on Dec. 1, 1992 and on Apr. 28, 1998 to Beshai et al.
Composite Star Network
To form a fully connected network, a moderate number, M, of edge nodes can be interconnected by a core node to form a basic star network, which has coverage limitation. For wide coverage, a composite star network interconnects the M edge nodes by a number of core nodes to form paths of adaptive capacity from each edge node to each other edge node. The number M is determined by the number of ports per core node. A path from a source node to a sink node consists of a channel from the source node to a core node and a channel from the core node to the sink node. The number M can be of the order of 1000, if the core node is an electronic space switch, or (currently) a relatively smaller number if the core node is an optical space switch.
The above composite star-like structure, which enables direct connection from a source node to a sink node, is limited by the port capacity of the core nodes; the number of edge nodes that can be supported by a composite star network is limited by the capacity of each of the bufferless core nodes. To realize higher capacity, a multi-dimensional structure can be used. The composite-star network is described in detail in Applicant U.S. patent application Ser. No. 09/286,431 titled “Self-Configuring Distributed Switch”, filed on Apr. 6, 1999, the specification of which is incorporated herein by reference.
Edge-Controlled Network
An edge-controlled network of an arbitrary structure in which paths of adaptive capacity are established from each source node to each sink node enhances scalability and performance while simplifying network control. An autonomous admission control mechanism, which relies neither on users' specifications of required capacity nor on users' declaration of traffic descriptors is described in U.S. Pat. No. 6,356,546, titled “Universal transfer method and network with distributed switch”, issued to Beshai on Mar. 12, 2002. A Universal Internet Protocol (UIP), which can work independently, or in conjunction with the autonomous admission-control mechanism, governs the communications among network nodes, which must be adapted to implement the UIP.
Agile Core
A core node controller selects paths through an associated core node and reconfigures the paths in response to dynamic changes in data traffic loads. Connection release and connection setup policies can be devised to increase occupancy variance among the space switches in each core node to reduce input/output mismatch and, hence, further facilitate reconfiguration. The reconfiguration functions of the edge nodes and the core nodes are coordinated to keep reconfiguration guard time at a minimum. The structure and control method permits the construction of a high capacity, load-adaptive, self-configuring switch that can be distributed geographically over a large area.
With adaptive circuit switching in the composite star network, a small proportion of the traffic can be routed through an intermediate edge node. In addition, a time-locking mechanism, to be described below, that enables coordination of the edge nodes and core nodes is required.
Applicant's U.S. patent application Ser. No. 09/671,140 filed on Sep. 28, 2000 and titled “Multi-grained Network” describes a network which includes edge nodes interconnected by core nodes having distinctly different granularities. The edge nodes switch multi-rate data traffic. The core may include core nodes that switch fixed-size data blocks, core nodes that switch channels or bands of channels, and core nodes that switch entire links. To simplify the control functions, the core nodes operate independently from each other. The network is fully meshed and the paths have adaptive capacities. The network capacity is then determined by the core node of least size. For example, if a core node has a dimension of 32×32, then a maximum of 32 edge nodes can be supported.
A network structure that maintains the simplicity and versatility of the multi-grained network while permitting expansion to much higher capacities is needed.